station was carried out in approximately 11 m of water just to the west of the shipping channel at approximately km 30 (Figure 10.1 and Figure 10.6). The observational period spanned moderately strong flood and ebb tides. The contours are based on a mix of underlying data resolution, ranging from 3 depth intervals at 0.5 to 1 h resolution for the turbulent shear and TSS data through 0.25 m by 0.5 h resolution for the volume concentration, D50, and salinity data to 0.25 m by 5 min resolution for the ADCP velocity contours. Many of the observed changes in the mid- and upper water column appear to be dominated by advection, as indicated most strongly by salinity. Comparing Figure 10.7 to Figure 10.6, it is apparent that the anchor station was located just upstream of a strong axial salinity convergence. This convergence was advected through the anchor station site during the flood tide and advected back out during the ebb tide of the anchor station. The patch of large, high volume concentration particles in the pycnocline noted in Figure 10.6 advected with the salinity structure, with little apparent dynamical relationship to physical energy. More interesting was the change in near-bottom concentrations and particle sizes, especially during the flood tide. Both TSS and volume concentration maxima corresponded to maximal tidal velocities and turbulent shears, indicating local resuspension. Higher D50 values during the resuspension period indicate that much of the resuspended material consisted of large, robust flocs, in spite of the high turbulent shears (up to 3.5 sec-1). However, maximum near-bottom D50 values occurred as the flood tide was decelerating and turbulent shear was decreasing, likely indicating the importance of local flocculation (or decreasing disaggregation).

Estimates of the space- and time-varying distributions of settling velocity derived from the LISST data of Figure 10.6 and Figure 10.7 are presented in Figure 10.8. These estimates were calculated using D50 and p, estimated from the LISST and Stokes law (Equation 10.2). As such, they use an unusual mix of mass and volume weighted bulk parameters, such that their absolute validity is not clear. However, they are useful as an indication of potential variability in ws, and also for comparison to other estimates of settling velocity below. Three results are apparent from these

0 0

Post a comment